1. Field of the Invention
This invention generally relates to a capacitive load driving device, and more particularly to a capacitive load driving device which applies a multi-level voltage to a capacitive load to drive the capacitive load.
2. Description of the Related Art
In an optical matrix switch, a multi-level high voltage is applied to an electro-optic effect device, and a refractive index of the electro-optic effect device is varied according to the applied voltage so that optical switching is carried out. The rise time and the fall time of the high-voltage pulse signal are set up to control the driving cycle of the optical matrix switch. High-speed switching is demanded for the optical matrix switch.
The electro-optic effect device is provided with the electrodes on both sides of the crystal. The electro-optic effect device is electrically regarded as a capacitor (or capacitive load), and a high voltage on the order of several hundred volts is applied between the electrodes.
FIG. 1 shows the composition of a conventional capacitive load driving device. As shown in FIG. 1, a control signal generator unit 1 which is composed of an ASIC (application-specific integrated circuit), such as FPGA (field programmable gate array), outputs a digital voltage control signal. After the digital voltage control signal output from the control signal generator unit 1 is converted into an analog signal by a D/A converter (DAC) 2, the analog signal is supplied to a voltage amplifier 3, and the voltage of the analog signal is amplified by the voltage amplifier 3. The amplified voltage signal is supplied to one end of a capacitive load 4. The other end of the capacitive load 4 is grounded.
Japanese Laid-Open Patent Application No. 2005-169737 discloses a capacitive load driving device as shown in FIG. 2. In the conventional capacitive load driving device of FIG. 2, a drive waveform signal output from a controller 5 is converted into an analog signal by a D/A converter 6, and the voltage of the analog signal output from the D/A converter 6 is amplified by a voltage amplifier circuit 7. The current of the amplified voltage signal output from the voltage amplifier circuit 7 is amplified by a current amplifier circuit 8. The amplified current signal output from the current amplifier circuit 8 is supplied to a piezoelectric element 9 which is a capacitive load.
Japanese Laid-Open Patent Application No. 47-037057 discloses a capacitive load driving device wherein a first current switch and a second current switch are connected in series via a pair of diodes, and a capacitive load is connected to the middle point of the pair of diodes. In this capacitive load driving device, the potential of the junction point of the second current switch and the pair of diodes is raised beforehand when charging the capacitive load. And when discharging the capacitive load, the potential of the junction point of the first current switch and the pair of diodes is lowered beforehand.
Japanese Laid-Open Patent Application No. 04-260089 discloses a capacitive load driving device which is adapted to quickly perform charging and discharging of a capacitive load by changing the voltage between the terminals of the capacitive load with a first current value, and thereafter driving the capacitive load with a second current value larger than the first current value.
In the case of the conventional circuit of FIG. 1, it is necessary that the output impedance when the voltage of the voltage amplifier 3 is varied at high speed is about 10 kΩ, in order to apply a multi-level voltage ranging from 0V to 100V to the capacitive load 4 having an electrostatic capacitance of some nanofarads (nF) at high speed. The time constant which is equal to a product of the output impedance of the voltage amplifier 3 and the capacitance of the capacitive load 4 is on the order of several ten microseconds. For this reason, there is a difficulty in performing the variable control of the voltage applied to the capacitive load 4 at a very high speed on the microsecond order.
In the case of the conventional circuit of FIG. 2, the current of the amplified voltage signal output from the voltage amplifier circuit 7 is amplified by the current amplifier circuit 8, and the amplified current signal output from the current amplifier circuit 8 is supplied to the capacitive load (piezoelectric element) 9. Thus, charging of the capacitive load at high speed is possible.
However, in order to vary the multi-level voltage applied to the electro-optic effect device at high speed (the applied multi-level voltage ranging between 0V and 400V), discharging of the capacitive load must be performed at high speed in accordance with the falling edges of the applied voltage.
In the case of the conventional circuit of FIG. 2, the change of the applied voltage is limited to one pattern. However, in a case in which there are many patterns including a pattern for changing the applied voltage from 400V to 0V, a pattern for changing the applied voltage from 400V to 380V, and so on, it is difficult to control the discharging of the capacitive load in accordance with the changes of the applied voltage in a wider range at high speed.